Check the SubmitCrashReport (DWORD) key in the registry and set the value to 0.
*HKEY_LOCAL_MACHINE\Software\Mozilla\Firefox\Crash Reporter\SubmitCrashReport: DWORD
*HKEY_CURRENT_USER\Software\Mozilla\Firefox\Crash Reporter\SubmitCrashReport: DWORD
You may need to check the HKLM\Software\Wow6432Node\ node on 64 bit Windows.
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*http://kb.mozillazine.org/Mozilla_Crash_Reporter